This invention relates to DC bus capacitor pre-charging systems and more specifically to a method and apparatus for use in controlling rectification of three phase AC supply voltages to allow pre-charging of a DC bus capacitor prior to full rectifier activity.
A typical power converter includes a rectifier stage and an inverter stage wherein the rectifier stage is provided between three phase AC power lines and a DC bus and the inverter stage is provided between the DC bus and a load. The rectifier, as the label implies, rectifies the three phase AC voltages to generate a DC voltage across the DC bus. The DC bus typically includes a DC bus capacitor that charges as the rectifier operates. The inverter receives the DC voltage and is controlled to change that DC voltage into three phase AC voltages that are provided to the load. Typically the inverter can be controlled to control the frequency and amplitude of the three phase voltages supplied to the load.
Inverters typically include a plurality of switching devices to convert the DC voltage to three phase AC voltages. As known in the power conversion industry, the inverter switches and or a load linked to an inverter can be damaged if they are exposed to excessive currents. One way to protect inverter switches/loads is to provide one or more fuses in the DC bus which open when excessive current passes there through.
When power is initially applied to a converter upon start up, if the DC bus capacitor is uncharged, capacitor appears as a short circuit at the DC bus and therefore, if the three phase AC power is immediately applied to the DC bus, excessive currents can cause the DC bus fuses to blow. For this reason, it is known that, prior to starting full rectification of three phase AC voltages and applying the rectified voltages to the DC bus, the DC bus has to be pre-charged to bring the DC bus potential up to a rated voltage level.
The power conversion industry has developed various ways to pre-charge the DC bus prior to full rectification activity. One way to pre-charge the DC bus is to place a parallel resistor and relay in series with the DC bus capacitor and to short out the resistor by closing the relay after the DC bus potential reaches the rated voltage value. The parallel resistor/relay solution works well with small drives where the cost of the relay and resistor is minimal.
Another way to pre-charge the DC bus prior to full rectification activity is to construct the rectifier stage using silicon controlled rectifiers (SCRs) and to control the turn on times of the SCRs to charge the DC bus over a period and in a controlled manner. The SCR solution works well in large drives where the cost of a relay required to short out the resistor can often exceed the additional costs associated with the SCR switches and a switch controller.